Most prior detectors of infrared imagers have pixels whose lateral spatial extent is defined by etching mesa structures whose sidewalls extend through the light absorbing layer. An example of a widely used dual-band detector structure is shown in FIGS. 1a-b. In this structure, the light absorbing regions for the two wavelength bands are stacked directly above each other and both of these light-absorbing layers are located completely within the same mesa as described by G. Destefanis, et al., “Bi-color and dual-band HgCdTe infrared focal plane arrays at DEFIR,” Proceedings SPIE Vol. 6206, 2006, p. 62060R, which is incorporated herein by reference in its entirety. Each pixel consists of two back-to-back photodiodes, with those diodes having, for example, N-on-P configurations. Each pixel has one independent contact and also has a contact that is common to the other pixels of the array. In general, the light-absorbing layer that absorbs the shorter-wavelength light is located closer to the incident light, which is the layer located closer to the substrate in the case shown in the figure. The two photodiodes of a pixel are electrically contacted and biased through the single contact located on the top of the mesa and through the array-common contact located on the substrate side of the structure. By switching the bias voltage for a pixel from a positive value to a negative value, it is possible to extract the photo-detected signal from one or the other P/N junction photodiode of a back-to-back pair.
The bias switched pixels can detect only one band at a time. Thus, the images for the two bands can be spatially offset from each other if the platform carrying the imager is moving rapidly compared to the switching speed for the band selection. Likewise, if an object being imaged is moving rapidly, the images formed for the two bands may indicate different spatial locations for that object. A dual-band imager that can provide outputs at both bands simultaneously is desirable for avoiding such image and/or object shifts.
FIG. 3 shows the cross-section structure and the material energy-band structure of another prior dual-band detector that has two outputs for each pixel as described by M. B. Reine, et al., “Simultaneous MW/LW dual-band MOVPE HgCdTe 64×64 FPAs,” Proceedings SPIE Volume 3379 (April 1998), pp. 200-212, which is incorporated herein by reference in its entirety. For this detector, the light-absorbing regions for the two wavelength bands are located within the same etched mesa and are aligned directly over each other. This detector is fabricated from a four-layer P-n-N-P structure, with an additional potential barrier for holes located at the n-N interface. In this structure, the longer-wavelength signal is obtained from one of the contacts for each pixel. However, the shorter-wavelength signal is obtained from the array common output. The other pixel-specific output provides the electron currents for both the longer wavelength and the shorter wavelength bands. Thus, a more complicated ROIC input circuit is needed to separate the signal currents for the two bands. There is a need for a compact dual-band detector that can simultaneously provide two outputs per pixel with those two outputs directly being the photo-currents for the two wavelength bands.
FIGS. 4a-c show the cross-section structure and the material energy-band structure of a prior detector that has aligned absorber regions for the two wavelength bands and also that can provide the photo-currents for those two wavelength bands at each detector pixel. This detector actually contains three mesas (as illustrated in FIGS. 4a-b) whose sidewalls extend completely through the longer-wavelength absorber layer of the structure as described in K. Kosai, et al., “Integrated LPE-grown structure for simultaneous detection of infrared radiation in two bands,” U.S. Pat. No. 5,559,336, issued 24 Sep. 1996, which is incorporated herein by reference in its entirety. These mesas enable the outputs of each pixel to be presented via three coplanar solder bumps to a readout circuit. One output is coupled to the collector layer that collects the photo-generated electrons from the longer-wavelength absorber layer. Another output is electrically connected to a contact made to the shorter wavelength absorber layer. These two outputs provide the photo-currents for the two bands. A third output is electrically connected to a contact made to the longer-wavelength absorber layer and its output is the combined photo-currents for the two bands. This detector has an N+-P-P+-N layer structure and its electronic band structure may be like the one shown in FIG. 4c as described by K. Kosai and G. R. Chapman, “Dual-band infrared radiation detector optimized for fabrication in compositionally graded HgCdTe,” U.S. Pat. No. 5,457,331, issued 10 Oct. 1995, which is incorporated herein by reference in its entirety. This band structure, which has a wide bandgap barrier to block the flow of electrons between the two light-absorbing layers, facilitates extraction of the electrons generated by absorption of the shorter-wavelength light through the second output and the extraction of the electrons generated by absorption of the longer-wavelength light through the first output. The holes generated by absorption of both the shorter-wavelength and the longer-wavelength light are extracted through the third output. A weakness of this prior detector is that with its three large mesas and thick absorber layers, the size of a pixel is likely 50 μm or larger. Thus, there remains a need to achieve a photo-detector array with simultaneous dual-band output that has sufficiently small pixel pitch to enable large-format imagers. Also, there is no provision for achieving a contact common to multiple pixels that can output a current due to both light-absorbing regions. That contact whose output is a current due to both light-absorbing regions would need to be made to the longer-wavelength light-absorbing layer, but the mesas form completely separated regions of the longer-wavelength absorber for each pixel and thus a contact common to multiple pixels cannot be made with this prior approach.
FIGS. 2a-b show the cross-sectional structure of a prior art dual-band detector in which each diode detector of a given pixel has a separate electrical contact, thereby allowing true simultaneous detection of light in the two wavelength bands, as described by P. Tribolet, et al., “Advanced HgCdTe technologies and dual-band developments,” Proceedings SPIE Volume 6940, paper 69402P, (2008) which is incorporated herein by reference in its entirety. For this prior structure, the lateral spatial extent of each pixel is defined not by an etched mesa but rather by a pair of PN junctions formed as a result of ion implantation. This known structure comprises a layer having wider bandgap material (the NIR barrier) located between the two light-absorbing layers that prevents electrical short-circuiting between the two diodes. This barrier blocks both the electrons and the holes. In this structure, the longer-wavelength absorbing layer (the LWIR or band-2 absorber) and the shorter-wavelength absorbing layer (the MWIR or band-1 absorber) are laterally contiguous and extend over multiple pixels. A deep via hole is etched through portions of the LWIR absorber layer in order to permit electrical contacts to be made to the PN junction for the MWIR absorber layer. In order to make contact for the return path of both diodes, the array also has other electrical contacts, located at the edge of the array, that abut both the shorter-wavelength absorbing layer and the longer-wavelength absorbing layer as shown in FIG. 2a. One weakness of this detector is that in each pixel of this detector, the detecting regions for the two bands are defined by the locations of the PN junctions for its two absorber layers. Since those two PN junctions are slightly offset from each other, as illustrated in FIG. 2b, the detections regions for the two bands also are offset from each other. This approach can achieve a pixel pitch as small as 20 μm, albeit with a small, fixed offset between the pixels for the two bands. This offset results in an undesirable spatial offset in the images produced for the two wavelength bands.
The prior dual-band detector arrays have generally been operated and tested at a device temperature of 77K. At this temperature, the dark-current noise is determined primarily by the thermal generation processes in the carrier-depleted junctions of the device. There is a need to achieve detectors that can operate at higher temperatures, such as 130-150K or even approaching 200K, and still have background limited noise performance. At these higher device temperatures, the thermal generation of carriers in the light-absorbing regions, especially for the longer-wavelength band, also can contribute to or even can dominate the dark-current noise. As a result, there is a need to reduce the volume of the longer-wavelength absorber material as well as reduce the depleted junction regions and still achieve high quantum efficiency for conversion from incident photons to output electrical carriers.
FIG. 5 shows a single band detector (albeit having very wide bandwidth) that can achieve high quantum efficiency with a reduced volume of its light-absorbing material. This detector contains multiple pyramid shaped features formed in each pixel, with those pyramids located on the side of the detector facing the incident light as described by D. Yap, et al., “Wide bandwidth infrared detector and imager,” U.S. Pat. No. 7,928,389, issued 19 Apr. 2011, which is incorporated herein by reference in its entirety. The pyramids are etched into a moderately thin light absorbing layer, with the overall thickness of that light-absorbing layer being roughly equal to the longest wavelength of the light to be absorbed. The pyramids are etched only partly through the light-absorbing layer so that there remains a physically continuous base region of the light-absorbing layer to permit the majority carriers to be conducted to electrical contacts formed at the edges of the detector array. This detector also contains mesas etched through the heavily doped collector or extractor layer of the P/N diode. The mesas are not etched into the main light-absorbing layer and the mesas face away from the incident light. The pixel-specific electrical contact for a given pixel is formed onto these collector mesas. The spatial extent of a given pixel is defined by the electrical contact made to these mesas. There can be more than one mesa for each pixel and there are multiple pyramids in each pixel.
For this detector, the light is incident from the side containing the pyramids rather than from the substrate side of the detector. Thus, the detector in FIG. 5 is unlike the prior detectors illustrated in FIGS. 1-4. Furthermore, the pyramid-shaped regions of the light-absorbing layer do not specifically define the extent of a pixel but rather they extend throughout the light-facing surface of the array. Instead, the pixel is defined by the extent of the electrical contacts made to the one or more collector mesas comprising a pixel and the electrical interconnection of those contacts. The electrical contact made to the light-absorbing layer of this structure is the common contact of the detector array.
FIG. 6 shows another single band detector (likewise having very wide bandwidth) that has only a thin layer of light-absorbing material. This detector contains multiple pyramid shaped features formed in each pixel, with those pyramids being transparent to the wavelengths of light to be absorbed as described by D. Yap and R. D. Rajavel, in U.S. application Ser. No. 13/372,366 “Wideband detector structures,” Filed on Feb. 13, 2012, which is incorporated herein by reference in its entirety. Those pyramids are located on the side of the detector facing the incident light. The pyramids are located above the thin and laterally continuous light absorbing layer. Although the thickness of the light-absorbing layer is much less than the 1/e absorption length for that material, it is possible to achieve quantum efficiency well above 80% over the entire wide bandwidth sensed by this detector. Deep dips in the absorption spectrum, at which wavelengths the absorption is greatly reduced, are avoided by forming multi-stepped mesas of the collector regions and/or oxide spacers of various thickness that separate the light-absorbing layer from a metal reflector at the backside of the device.
FIG. 7 shows a dual-band detector that has a reduced volume of the light-absorbing material for the longer-wavelength band as described in U.S. application Ser. No. 13/37036,403 “Infrared Detector,” Filed on Feb. 28, 2011, which is incorporated herein by reference in its entirety. Light is incident from the substrate side of that device, which is illustrated in FIG. 7. For that detector, the growth substrate is preferably thinned but does not need to be removed completely. Pyramids are formed in the thinned substrate and they serve to improve the coupling of light into the light absorbing regions. The light absorbing regions for the longer-wavelength band are laterally separated from each other and are surrounded by voids that may be filled with a transparent, low-refractive-index material. A metal reflector at the backside of the detector acts in combination with these trapezoidal shaped light-absorbing regions to achieve efficient trapping and absorption of the light. The light-absorbing region for the shorter-wavelength band is a laterally contiguous layer. This prior detector has a single electrical output per pixel as well as an array common output. Thus, the dual-band operation is achieved by switching the bias voltage applied to a detector pixel.
Most of the prior dual-band detectors have a separate PN diode for detecting the light of each band, with a wide bandgap barrier that electrically isolates these two PN diodes by blocking the flow of both electrons and holes (as shown in FIGS. 2 and 4). Such back-to-back PN diode configurations result in devices that require much larger area. The dual-band detector shown in FIG. 7, and also the prior detector shown in FIG. 3, has a barrier between the two light-absorbing regions for the two bands, with that barrier blocking one electrical carrier but not the other carrier. These prior detectors essentially have one electrical output that provides the signal for one band and a second electrical output that provides a combined signal for both bands. This configuration requires a more complicated front-end circuit for the read out.
In view of the limitation in prior art, a need exists for improved detectors.
In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale.